This invention relates to a wide converter lens system for both underwater and above-water photography. Explained more in detail, a lens system adapted to be attached in front of the main lens of a camera in order to convert its effective focal length (or the field angle) is often referred to as a front converter, or simply as a converter. This invention relates to such a lens system adapted particularly to convert the effective focal length of the camera lens so as to make it function as a wider-angle lens, referred to as a wide converter, attachable to a water-proof camera housing enclosing the camera therein. More particularly, the invention relates to a multi-functional system of this kind especially suited for underwater photography but also usable under above-water conditions.
There has recently been a remarkable progress in the technology of electronic apparatus such as digital cameras and video cameras having a CCD image capturing device and optical instruments such as cameras for a 35 mm film and cameras for a new film having a magnetic memory part. Progress in miniaturization and multiple functionality has also been remarkable. For underwater photography, divers carry cameras of these kinds enclosed inside a waterproof housing. In spite of the general insufficiency of available light in the case of underwater photography, it is becoming possible to produce equally high-quality underwater pictures as pictures taken under an above-ground condition, thanks to the recent improvement in the photographic instruments.
Most popularly used cameras, however, are designed as a land camera with a fixed focal length of 38 mm or 45 mm somewhat on the wide-angle side. Even cameras with a zoom capability are usually designed as a land camera with the field angle (or the light-receiving angle) of the lens selected to cover a portion of the so-called wide-angle region and a portion also of the so-called telephoto region such as the range of 35-70 mm. When such a camera is used in water, it cannot function as a wide-angle camera. In other words, since the index of refraction of water is 1.33 and considerably higher than that of air (=1.00), a lens with a fixed focal length of 35 mm or 45 mm in the above-water condition (or in air) will function in water as a lens with a focal length of 50 mm or 65 mm, and a zoom lens of focal length 35-70 mm in air will function as a zoom lens of focal length about 50-100 mm in water. In either case, the lens will not function as a wide-angle lens.
With such a lens without a wide-angle capability, it is not possible to photograph a dynamic underwater scenery or a large group of fish. A wide conversion lens system is a peripheral device for such an application, being a kind of supplementary lens system and adapted to be set in front of a camera lens to vary the angle of its field of vision in a simple manner, enlarging it into the wide-angle side.
Conventionally, both lens makers and users regarded wide converter lens systems merely as one of simple single-function camera accessories capable of easily expanding the field of vision into a wide-angle region and there was not available any carefully designed wide conversion lens system of a high quality. Thus, prior art wide conversion lens systems were under a strict price limitation and were of a simple lens structure such that the image and color qualities were often adversely affected in peripheral areas. Moreover, wide conversion lens systems for underwater and above-water uses were being developed and produced as different single-function devices.
In the meantime, with the recent development in digital cameras and video cameras, cameras themselves have become high-precision electronic instruments requiring delicate handling because they are easily damaged, for example, by a small amount of water or sand. Thus, cameras enclosed in a waterproof housing have come to be used for underwater photography and also heavy-duty cameras protected against dust and water droplets have come to be used as land cameras.
It is therefore an object of this invention to provide a lens system serving as a wide converter with which a larger field angle can be attained even in an underwater condition.
It is another object of this invention to provide such a lens system for allowing a camera to be used as a wide-angle camera both under underwater and above-ground conditions.
A wide converter lens system embodying this invention, with which the above and other objects may be accomplished, is characterized as comprising what is herein referred to as a converter which is an assembly of two or more specified kinds of lens units and a forwardly protruding meniscus lens to be attached in front of this assembly. The expression xe2x80x9clens unitxe2x80x9d will be used herein to indicate both a single lens and two or more lenses pasted together to form a single combination. The converter is detachably attachable to the camera, that is, to the watertight underwater housing containing the camera in a non-airtight, non-watertight manner so as to increase the field angle of the camera. The assembly of lens units includes at least a forwardly disposed one having a negative refractive power and a backwardly disposed one having a positive refractive power. According to a preferred embodiment, the former is a plano-concave lens or lens unit with its flat surface facing forward away from the camera and the latter is a plano-convex lens or lens unit with its flat surface facing the camera such that the converter can used equally conveniently in the above-water condition. Between these two lens units, the converter may further include any number of lenses or lens units for the purpose of correcting different kinds of distortions and aberrations. The meniscus lens is usually a concave lens and is provided with a mechanism for detachably attaching itself to the front part of the frame of the converter in a watertight manner and serves to further increase the field angle of the camera in water. Such an attaching mechanism may be either on the meniscus lens itself or on the frame to which it is to be attached.
As a variation, a transparent plate with flat surfaces may be provided between the camera and the converter, attached to the converter in an airtight and watertight manner and adapted to be attached to the camera housing in a non-watertight and non-airtight manner. In other words, a meniscus lens according to this invention may be designed to be attachable to a converter of this type having a flat transparent plate in order to further increase the field angle of the camera.
The plano-concave lens or lens unit of the converter need not be strictly plano-concave as long as its front surface has a radius of curvature sufficiently large such that the converter can serve as a wide converter in water. Similarly, the meniscus lens need not be concave, depending on the specifications of the converter, as long as the radii of curvature of its surfaces are so designed that the field angle of the camera can be increased to a desired angle, when the meniscus lens thus designed is attached to the converter for an underwater use.